The present invention relates to electron multipliers which employ ion feedback and more particularly to such devices having means for preventing the ions from striking the electron emissive regions of the dynodes of the multiplier.
Various types of cathodoluminescent image display devices have been recently suggested employing ion feedback electron multipliers as electron sources. Such devices incorporate flat electron multipliers, each formed by a dynode chain having a cathode at one end and some form of cathodoluminescent screen at the other end. In these devices, the electrons in the first multiplier stages are amplified forming many electrons in the final stage, which in turn strike residual gas molecules in the atmosphere of the device converting the molecules to positive ions. These ions travel to the cathode which is coated with a secondary emissive material. The ions bombard the cathode emitting additional electrons which travel to the first stages of the electron multiplier completing a feedback loop.
The device can achieve self-sustained electron emission if the gain of the feedback loop gain exceeds one. In this case, the device may be turned on and off by controlling the electrical potentials applied to the dynodes or the cathode so as to switch the feedback loop gain above or below one respectively. It has been found, however, that the ions can strike the dynodes in the electron multiplier, as well as the cathode. When the ions strike the dynodes, additional electrons are given off, creating problems with respect to the on-off control of the device. The control of the device may be lost if the ions strike and create secondary electrons on higher stage dynodes than the dynode or the cathode whose potential has been changed. These secondary electrons will not be suppressed by the switching of lower dynode or cathode potentials. Thus a sustained electron emissive feedback loop may exist in the final stages which can not be turned off by potential changes at the cathode or the early multiplier stage dynodes. This control problem is most serious in certain applications where one desires to control the device only through potential changes on the cathode. In this situation, ion bombardment of the first multiplier stages can cause loss of cathode control. Such control problems can be avoided if this ion bombardment of the dynodes can be eliminated.